1. Field of the Invention
The present invention relates to a process for growing a silicon single crystal based on the Czochralski method, including addition of a dopant, more particularly, to a process for growing a silicon single crystal in which the concentration of a dopant in a melt can be regulated within a predetermined range even when a dopant having a high evaporation speed is added, and a single crystal having small variation in specific resistance can be produced.
2. Description of the Prior Art
There are a variety of processes for producing a single crystal and among other things, the Czochralski method is widely employed for pulling a silicon single crystal in view of potentiality of industrial mass production. In the Czochralski method, a silicon single crystal is grown in a closed, atmosphere-controlled metal chamber in which a crucible containing a melt of polycrystalline silicon serving as a crystal source is placed. The tail end of a seed crystal, which is set on the top of pulling apparatus including a wire, is contacted to the surface of the melt. When the seed crystal is pulled upward, the melt is solidified onto the tail end thereof, to thereby grow a single crystal.
In the growth of a silicon single crystal, the shape of the pulled single crystal comprises a necking portion which is connected to a seed crystal and has a constricted diameter of a cylinder; a cone portion having a cylindrical diameter gradually increasing; and a main body serving as a wafer product after crystal growth. Specifically, a process for growing a single crystal comprises the steps of: forming a necking portion to render a produced single crystal dislocation-free; forming a shoulder portion serving as a cone portion to ensure a sufficient diameter of the produced single crystal; and forming a main body by pulling a single crystal with maintaining the above diameter.
Conventionally, in order to grow a single crystal having a target specific resistance, a predetermined amount of dopant such as phosphorus (P), boron (B), arsenic (As), or antimony (Sb) has been incorporated into a melt from an early stage of pulling. In general, a dopant having a relatively low vaporization speed such as P or B is added to a crystal raw material and the mixture is molten as a preliminary step for single crystal growth. However, when a dopant having a relatively high vaporization speed such as As or Sb is added to a crystal raw material from a melting step, a large amount of added dopant is vaporized during the melting step, to thereby affect a doping efficiency. Thus, such a dopant is added to a melt contained in a crucible after a crystal raw material is molten.
FIG. 1 (PRIOR ART) is a flow chart showing a dopant addition step during a conventional process for growing a silicon single crystal. As described above, a dopant is added to a melt contained in a crucible after a crystal raw material is completely molten. After the dopant is added, a seed crystal is brought into contact to a melt. This step is referred to as "the seed crystal contact technique." In general the seed crystal contact technique is carried out a predetermined time after melting of a crystal raw material, since the temperature of the melt just after melting of the raw material is higher than the melting temperature of silicon and variation in temperature is locally significant to thereby induce large variation in temperature of the entirety of the melt. The seed crystal contact technique is a method for stabilizing the surface temperature of a melt comprising the steps: bringing into contact a seed crystal with a melt so as to form a meniscus; assuming the surface temperature through observation of the meniscus; and controlling the output electric power of a heater to thereby regulate heat input to the melt.
When the seed crystal contact technique is completed and the melt contained in the crucible is sufficiently stabilized, growth of a single crystal is initiated. Thereafter, as described above, a necking portion and a cone portion having a cylindrical diameter gradually increasing are successively formed and a main body having a diameter corresponding to that of a wafer product is formed through pulling.
As described above, the seed crystal contact technique is carried out in order to regulate the temperature of a melt. However, there are a variety of external factors of variation in temperature during actual operations, and the time which is required to complete stabilization of the melt to thereby allow a single crystal to grow is not constant. Therefore, the time from addition of a dopant to start of single crystal growth, i.e., the formation of a necking portion significantly varies. Under such circumstances, especially when a dopant having a high speed of vaporization from a melt such as As or Sb is added, the concentration of the added dopant in the melt contained in a crucible significantly varies due to variation of the time from addition of the dopant to the formation of a necking portion. Therefore, some single crystals, which do not ensure a target specific resistance, are disadvantageously produced among the silicon single crystals grown from a dopant-added melt.
In accordance with recent trends of semiconductor devices having a high-quality function, specific resistance of a device substrate is strictly regulated. Thus, an important problem that variation in the time from addition of the dopant to initiation of single crystal growth induces variation of a dopant concentration in the melt becomes critical and must be solved as quickly as possible. In connection with this problem, it has been proposed that the time from addition of the dopant to initiation of single crystal growth be constantly fixed to a longer time in consideration of variation in the above time. However, this method is not advantageous in view of production efficiency. Actually, the time from the addition of the dopant to the initiation of single crystal growth varies from 1.0 to 12.0 based on a minimum unit time.
In order to suppress variation in the time from the addition of the dopant to initiation of single crystal growth, the timing for adding dopant has been determined based on the data obtained through optical measurement of the temperature of a melt. However, this measurement has a limitation in accuracy and a poor reliability when the temperature of the melt is optically detected by use of an optical thermometer such as a mono- or bi-color thermometer. Therefore, even when the dopant is added based on the data obtained through the above optical measurement, the effect of such addition on suppression of variation is insufficient.